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DIVISION OF GEOLOGY AND MINERAL RESOURCES

Tsunamis

A tsunami is a set of abnormally large oceanic waves characterized by an extremely long wavelength and wave period.  These waves are generated when a huge volume of water is rapidly displaced, and they can travel a hundred miles an hour in the open ocean, building up as they reach land, crashing onto the shoreline as a wall of water up to a fifty feet high.  Tsunamis are occasionally referred to as “tidal waves,” but that term is misleading because they have nothing to do with tides; they are more properly called “seismic sea waves” because the most common triggering event is an earthquake.  However, volcanic eruptions, underwater landslides, or large meteor impacts can also generate a tsunami.  

Tsunamis are among the Earth’s deadliest natural disasters, primarily because they strike without warning.  Typically, a tsunami comes ashore like a very fast rising tide which quickly builds into a wall of water that can demolish coastal communities within minutes.  As the sea recedes, it leaves behind piles of debris, dead bodies, and contaminated drinking water supplies.  Since 1850, tsunamis have been responsible for the loss of over half a million lives and untold billions of dollars in damage. 

Tsunami means “harbor wave” in Japanese and the great majority of tsunamis occur in the Pacific Ocean as a result of the high seismic activity along the circum-Pacific “Ring of Fire.”  However, seismically generated sea waves are not unknown in the Atlantic.  On October 11, 1918, a magnitude 7.5 earthquake occurred in the Puerto Rican subduction zone and created a twenty-foot-high wave that crashed into the Puerto Rican coast.  Lesser tsunamis struck the Virgin Islands in 1867 and the Dominican Republic in 1946.  The greatest Atlantic tsunami in historic times happened on November 1, 1755, when the Portuguese capital of Lisbon was flattened by an earthquake.  As the city burned, people who sought refuge on the banks of the Tejo River were swept away by huge waves produced by the earthquake.  About 60,000 people lost their lives.  A fifty-foot wall of water struck the Madeira Islands, and tsunami waves reached as far as Ireland and the Lesser Antilles.

Most tsunamis are caused by large, shallow focus earthquakes of magnitude greater than 7.0, and these types of quakes off the mid-Atlantic coast are unknown in historical times.  However, farther distant sources could also create a tsunami here, such as an ultra-high-magnitude earthquake in the Puerto Rican subduction zone, or perhaps the collapse of the Cumbre Vieja volcano in the Canary Islands.  Non-seismic events also could trigger a large sea wave impacting Virginia: for example an explosion of a vessel in Hampton Roads similar to the December 6, 1917, explosion that obliterated Halifax harbor and unleashed a fifty-foot-high tsunami.  The recently discovered impact of a comet or meteor that occurred about 35 million years ago near the mouth of the present Chesapeake Bay undoubtedly triggered a large tsunami that reached far inland, but there have been no studies to determine the extent of this event.  Another possible (though quite speculative) source of tsunami waves could be the vast amount of methane now buried as ice in coastal shelf sediments underneath the Atlantic Ocean; if the methane warmed up and quickly sublimated from ice to gas, the gas could burst to the surface and create a sea wave.

An underwater landslide could also trigger a tsunami.  On September 3, 1821, as a Category Four hurricane moved across the Delmarva Peninsula, sandbars were exposed from the recession of the Atlantic Ocean along the barrier island of Assateague.  A witness stated that suddenly “a monstrous wall of inky waters rushed with the speed of lightning toward the island.  It struck Assateague, and in a moment half the land was a waste of seething foam and tossing pine trunks; the next instant it struck Chincoteague…”  It has been suggested that the action of the water was not typical for a hurricane, and may have resulted from an underwater landslide that generated a tsunami.

Virginia has over three thousand miles of estuarine shoreline, but there have been no studies on the effect of a tsunami on tidal estuaries.  Areas are at greater risk if they are less than twenty-five feet above sea level and within a mile of the shoreline. Thus, the Eastern Shore and the cities of Chesapeake, Virginia Beach, Hampton, Newport News, Portsmouth, and Norfolk are particularly vulnerable. 

The Virginia Department of Environmental Quality participates in the National Oceanic and Atmospheric Administration (NOAA) Tsunami Ready program, and in January 2006, Norfolk became the first major East Coast city to be officially recognized by NOAA as “Tsunami Ready.”  The Virginia Department of Emergency Management (VDEM) has no program designed specifically for tsunamis.  However, VDEM has created storm surge maps for the coastal areas based on hurricane categories and data from the partially completed Virginia Hurricane Evacuation Study, a joint effort by VDEM, the Federal Emergency Management Agency (FEMA), the U.S. Army Corps of Engineers and coastal localities.  These maps might be useful in predicting the effect of a tsunami.

 

What should I do in the event of a tsunami?

The actual vulnerability of Virginia to a devastating sea wave is unknown.  Geologic investigations are necessary to identify prehistoric tsunamis and answer questions concerning frequency and extent.


Web sites:

USGS general public hazards information

USGS tsunami research

NOAA’s tsunami research, general description of tsunamis

NOAA’s TsunamiReady web site

FEMA tsunamis web site

Virginia Places web site discusses possibility of a tsunami in Virginia

 

References: Lockridge, Patricia A., Lowell S. Whiteside, and James F. Lander, 2002, Tsunamis and tsunami-like waves of the Eastern United States: Science of Tsunami Hazards, International Journal of the Tsunami Society, Honolulu, Hawaii, Vol. 20, No. 3, p. 120-144.

Maretzki, S., Grilli, S., and Baxter, C.D.P, 2007, Probabilistic SMF tsunami hazard assessment for the upper East Coast of the United States: Advances in Natural and Technological Hazards Research. Vol. 27 p. 377-385. Accessible online: http://www.oce.uri.edu/~grilli/Maretzki-Grilli-Baxter-07.pdf

Maretzki, S. 2006, Numerical simulation of tsunami hazards maps for the US East Coast: Masters Thesis, Department of Ocean Engineering, University of Rhode Island, 187 pp.

Pyle, Howard, 1877, Chincoteague; The Island of Ponies: Scribner's Monthly, Vol. XIII, No. 6, April, 1877, p. 737-746. Accessible at Cornell University Library online: http://ebooks.library.cornell.edu/cgi/t/text/pageviewer-idx?c=scmo;g=moagrp;xc=1;q1=chincoteague;rgn=full%20text;cite1=pyle;cite1restrict=author;view=image;cc=scmo;seq=0743;idno=scmo0013-6;node=scmo0013-6%3A1

United States Geological Survey, 2006, Tsunami Hazards – A National Threat: USGS Fact Sheet 2006-3023 February 2006.

Ward, S. N. and Day, S. (2001). Cumbre Vieja Volcano—potential collapse and tsunami at La Palma, Canary Islands: Geophys. Res. Lett., 28, 397-400.